Module No E03 Electrical Test Equipment Basic

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Capability Improvement Dept.2004 For Internal Use Only

P ET RO N A S

G A S

TRAINING MODULE

ELECTRICAL

(BASIC)

TITLE : TEST

INSTRUMENTS

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Electrical test instruments Module 03 (Basic) Page 2of 55

DUTY NO 15: TEST INSTRUMENTS

OBJECTIVES

Upon completion of this module, the technician would be able to demonstrate knowledge and understanding on the following:

1. Safety precautions in test & measurement

2. Basic construction and operation of Multimeter (Digital & Analog) 3. Functional selection, setting of Digital Multimeter

4. Use of digital multimeter and test procedures. 5. Theory of Insulation measurement

6. Basic construction and operation of Insulation resistance tester 7. Functional selection, setting of Insulation resistance tester 8. Use of digital Insulation resistance tester and test procedures. 9. Basic construction and operation of Clamp on Ammeter 10. Functional selection, setting & use of Clamp on Ammeter 11. Theory of Earth Resistance measurement

12. Function and operation of Earth Resistance tester 13. Preparation and set-up of Earth Resistance tester 14. Function and operation of Loop tester and RCD tester 15. Preparation and set-up of Loop tester and RCD tester

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TABLE OF CONTENT

1.0.0 Introduction... 5

2.0.0 Safety... 6

2.1.0 Causes of Electrocution...8

2.2.0 Use of High voltage protection equipment ...8

2.2.1 Clearances ...9

2.3.0 Section 2 Safety of BS 6626: 1985 ...10

2.3.1. Responsibility ...10

2.3.2 Rules or procedure for safe systems at work ...10

2.3.3 Isolation and access for maintenance ...11

2.3.4 Preparing for maintenance work ...12

2.3.5 Fire extinguishing equipment ...13

2.3.6 Testing...14 2.3.7 Disposal of scrap...14

3.0.0 Multimeter ... 15

3.1.0 Analog Multimeter...16 3.2.0 Digital Multimeter ...17 3.2.1 Voltage Measurements...20 3.2.2 Current Measurements...21 3.2.3 Resistance Measurements...21

3.3.0 Digital vs. Analog Multimeters...22

3.4.0 Safety Precautions...22

4.0.0 Insulation resistance tester (Megohmmeter) ... 23

4.1.0 Analog Megohmmeter ...23

4.2.0 Digital Megohmmeter ...24

4.3.0 Insulation resistance test...26

4.3.1 Components of DC leakage current ...26

4.3.2 Determining The Polarization Index ...27

4.3.3 Correction for Winding Temperature ...28

4.3.4 Insulation Contamination...29

4.4.0 Insulation resistance test methods...29

4.4.1 Spot testing...29

4.4.2 Step voltage test ...30

4.4.3 Time resistance test ...30

4.6.0 Insulation resistance (IR) test on Cables ...31

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4.8.0 Insulation resistance (IR) test on Motor/Generator...32

5.0.0 Clamp on meter ... 34

5.1.0 Theory of operation of AC Clamp on meter ...35

5.2.0 Theory of operation of AC/DC Clamp on meter...36

5.3.0 Specifications of Digital Clamp on meters...37

5.4.0 Advantages of Digital Clamp on meter...37

5.5.0 Advantages of Digital over Conventional Type ...37

5.6.0 Applications of Digital Clamp on meters...38

5.7.0 Clamp on meter operations (Fluke model 321/322)...38

6.0.0 Earth tester, Loop tester & Residual Current Device (RCD)

tester... 42

6.1.0 Earth resistance ...43

6.2.0 Principle of Earth resistance testing ...44

6.3.0 Earth resistance test methods ...46

6.4.0 Earth Loop resistance ...47

6.5.0 Earth Loop resistance test...48

6.6.0 Digital Earth Loop resistance tester from Megger (L T5 and L T6) ..48

6.7.0 Applications & Use of Earth Loop resistance tester...49

6.8.0 Residual Current devices (RCDs) ...50

6.8.0 Residual Current devices (RCDs) ...51

6.9.0 Testing of RCDs ...51

6.10.0 Digital RCD tester from Megger (CBT3 and CBT4)...52

7.0.0 Attachments... 55

7.1.1 Fluke multimeter manual...55

7.12 Megger Manual BM80...55

7.13 Fluke Clamp on meter Instruction sheet...55

7.14 Megger Digital Earth Tester...55

7.15 Megger Digital Loop Tester...55

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1.0.0 Introduction

Testing is performed to verify the integrity of electrical systems. Most of these tests are non-destructive in nature and can be used to provide a complete look at the status and age of the equipment. In this module, we will concentrate on the following electrical tests instruments:

Multimeter (Digital & Analog) Insulation resistance tester Clap on Ammeter

Earth resistance tester

Earth loop impedance & RCD tester

In this module included the some specific OEM instruction manuals for the above mentioned test set. The module basically prepared to train the technicians to read and interpret the OEM manuals of the test instrument, which is the required to perform the some of the tasks prescribed in POSS under duty no. 15. In the process, it covers the basic underpinning knowledge required to perform the required tasks. For gaining the expertise in the activities, detail study of the “Operation & Maintenance” manual of respective test instrument and hands on experience is necessary.

Note: Testing of electrical distribution equipment requires experience and an understanding of the hazards involved. The test equipment used at your workplace may be different or from different manufacturer than the one discussed in the module. You are therefore advised to read and understand the manufacturer's specifications/ guidelines and make yourself well conversant before attempting any testing work or operating the test equipment.

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2.0.0 Safety

In the interest of safety, all test equipment should be inspected and tested before being taken to the job site. There is no need to get to the job and find that the test equipment does not work.

A thorough visual inspection (i.e., checking for broken meters or knobs, damaged plugs, or frayed cords) is important.

Always perform an operational check. For example:

On an ohmmeter, short the probes and ensure that you can zero the meter. A voltmeter can be checked against an AC wall receptacle or a battery.

If a meter has a calibration sticker, check to see if it has been recently calibrated. For precise measurements, a recently calibrated meter is a more reliable instrument.

Every person who works with electrical equipment should be constantly alert to the hazards to which personnel may be exposed, and should also be capable of rendering first aid. The hazards are electric shock, burns, and related hazards.

Safety must be the primary responsibility of all personnel. The installation, maintenance, and operation of electrical equipment enforce a strict safety code. Carelessness on the part of the technician or operator can result in serious injury or death due to electrical shock, falls, burns, flying objects, etc. When an accident has occurred, investigation almost invariably shows that it could have been prevented by the exercise of simple safety precautions and procedures. Each person concerned with electrical equipment is responsible for reading and. becoming thoroughly familiar with the safety practices and procedures contained in all safety codes and equipment technical manuals before performing work on electrical equipment. It is your personal responsibility to identify and eliminate unsafe conditions and unsafe acts which can cause accidents.

You must bear in mind that de-energizing main supply circuits by opening supply switches will not necessarily de-energize circuits in a given piece of equipment. A source of danger that has often been neglected or ignored, sometimes with tragic results, is the input to electrical equipment from other sources, such as back-feeds. Moreover, the rescue of a victim shocked by the power input from a back-feed is often hampered because of the time required to determine the source of power and isolate it. Therefore, turn off all power inputs before working on equipment, tag and lock out, then check with an operating tester to be sure that the equipment is safe to work on.

Take the time to be safe when working on electrical circuits and equipment. Carefully study the schematics and wiring diagrams of the entire system, noting what circuits must be de-energized in addition to the main power supply. Remember, electrical

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equipment commonly has more than one source of power. Be certain that all power sources are de-energized before servicing the equipment. Do not service any equipment with the power on unless absolutely necessary. Remember that the 115V power supply voltage is not a low, relatively harmless voltage but is the voltage that has caused more deaths than any other medium.

Safety can never be stressed enough. There are times when your life literally depends on it. The following is a listing of common safety precautions that must be observed at all times:

Use only one hand when turning power switches on or off.

Keep the doors to switch and fuse boxes closed except when working inside or replacing fuses.

Use a fuse puller to remove cartridge fuses after first making certain that the circuit is dead.

Ensure that you are qualified and authorised to work on an electrical circuit (LV or HV). Do not work with energized equipment by yourself; have another person (safety

observer) that is qualified in first aid for electrical shock present at all times.

The person stationed nearby should also know which circuits and switches control the equipment, and should be given instructions to pull the switch immediately if anything unforeseen happens.

Always be aware of the nearness of high-voltage lines or circuits. Use rubber gloves where applicable and stand on approved rubber matting. Not all rubber mats are good insulators.

Comply to PTW and inform those in charge of operations as to the circuit on which work is being performed.

Keep clothing, hands, and feet dry. When it is necessary to work in wet or damp locations, use a dry platform and place a rubber mat or other nonconductive material on top of the wood. Use insulated tools and insulated flashlights of the moulded type when required to work on exposed parts.

Do not work on energized circuits unless absolutely necessary.

All power supply switches or cut-out switches from which power could possibly be fed must be secured in the OPEN (safety) position and perform LOTO.

Never short out, tamper with, or block open an interlock switch.

Keep clear of exposed equipment; when it is absolutely necessary to work on it, use only one hand as much as possible.

Avoid reaching into enclosures except when absolutely necessary. When reaching into an enclosure, use rubber blankets to prevent accidental contact with the enclosure.

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Do not use bare hands to remove hot vacuum tubes from their sockets. Wear protective gloves or use a tube puller.

Use a shorting stick (discharge rod) to discharge all high-voltage capacitors.

Make certain that the equipment is properly grounded. Ground all isolated and discharged circuits of the equipment under test to prevent accidental charging.

Turn off the power before connecting alligator clips to any circuit.

When measuring circuits over 440V, do not hold the insulated test probes with bare hands.

2.1.0 Causes of Electrocution

Unsafe Acts:

a Accidentally slipping with wrenches, screwdrivers, etc., while working on or near electrical equipment with live parts (over 50 volts)

a Switching off the wrong circuit and then failing to verify that the circuit is de-energized before beginning work.

a Failing to implement lock-out/tag-out procedures or use adequate protective equipment.

a Use of noninsulated tools.

a Wearing metal jewelry while working on live circuits.

a Using instruments/meters/tools not designed for the system voltage. a Non-electrical personnel working too close to live equipment (e.g. power

lines), usually with cranes or lifting equipment or handling metallic material.

Unsafe conditions:

a Improper grounding, loose connections, defective parts, ground faults, unguarded live parts or faulty insulation in equipment.

a Inadequate maintenance.

a Hazardous environments, e.g. corrosive or flammable atmosphere, wet or damp locations.

a Inadequate working clearance.

2.2.0 Use of High voltage protection equipment

Anyone working on or near energized circuitry must use special equipment to provide protection from electrical shock. Protective equipment includes gloves, leather sleeves, rubber blankets, and rubber mats. It should be noted that this electrical protective equipment is in addition to the regular protective equipment normally required for maintenance work. Regular protective equipment typically includes hard hats which are

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rated for electrical resistance, eye protection, safety shoes, and long sleeves.

Gloves that are approved for protection from electrical shock are made of rubber. A separate leather cover protects the rubber from punctures or other damage. Gloves are rated as providing protection from certain amounts of voltage. Whenever an individual is going to be working around exposed conductors, the gloves chosen should be rated for at least as much voltage as the conductors are carrying. Rubber sleeves are used with gloves to provide additional protection. The combination of sleeves and gloves protects the hands and arms from electrical shock.

Rubber blankets and floor mats have many uses. Blankets are used to cover energized conductors while work is going on around them. They might be used to cover the energized main busses in a breaker panel before you begin working on a deenergized breaker. Rubber floor mats are used to insulate workers from the ground. If a worker is standing on a rubber mat and contacts an energized conductor, the current cannot flow through the body to the ground, so the worker will not get shocked.

2.2.1 Clearances

Adequate clearances are to be maintained between energized and exposed conductors and personnel. Where DC voltages are involved, clearances specified shall be used with specified voltages considered as DC line-to-ground values.

If adequate clearances cannot be maintained from exposed live parts of apparatus in the normal course of free movement within the area during test, then access to that area shall be restricted by fences and barricades. Signs clearly indicating the hazard shall be posted in conspicuous locations. This requirement applies to equipment in service as well as to equipment to which test voltages are applied.

Whenever there is any question of the adequacy of clearance between the specific area in which work is to be done and exposed live parts of adjacent equipment, a field inspection shall be made by management representatives of the group involved before starting the job. The result of this inspection should be to outline the protection necessary to complete the work safely, including watchers where needed.

An important piece of information is the minimum distance allowed when working near energized electrical circuits, because large voltages can arc across an air gap. Personnel must maintain a distance that is greater than that arc distance. This is especially true when using a hot stick to open a disconnect.

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These distances are listed in Table 1.

Voltage Range

Minimum Working and (Phase-to-Phase) Kilovolts Clear Hot Stick Distance

2.1 to 15 2 ft. 0 in. 15.2 to 35 2 ft. 4 in. 35.1 to 46 2 ft. 6 in. 46.1 to 72.5 3 ft. 0 in. 72.6 to 121 3 ft 4 in. 138 to 145 3 ft. 6 in. 161 to 169 3 ft. 8 in. 230 to 242 5 ft. 0 in. 345 to 362 *7 ft. 0 in. 500 to 552 *11 ft. 0 in. 700 to 765 *15 ft. 0 in.

* For voltages above 345 kV, the minimum working and clear hot stick distances

may be reduced provided that such distances are not less than the shortest distance between the energized part and a grounded surface.

Table 1. OSHA Working And Hot Stick Distances At Various Voltages

2.3.0 Section 2 Safety of BS 6626: 1985

2.3.1. Responsibility

Electrical equipment should be regarded as being capable of giving rise to danger, not necessarily of an electrical nature, and it is essential that all persons responsible for electrical work make themselves acquainted with the relevant statutory requirements. A list of some relevant publications is included in the foreword, All persons concerned with the maintenance of equipment should conduct themselves in accordance with the provisions of the statutory requirements and take reasonable care for the health and safety of all those carrying out the work, and others who may be affected by their acts or omissions at work.

A notice giving instructions for the treatment of persons suffering from electric shock should be affixed in a prominent position in the vicinity in which work on electrica1 installations will be carried out. It is strongly recommended that all electrical maintenance personnel be trained in the application of resuscitation and know how to summon medical help.

2.3.2 Rules or procedure for safe systems at work

It is recommended that in all premises, the employer or occupier should formulate and update, as needed during the life of the equipment, a set of safety rules or procedures,

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appropriate .to the type of electrical installation, covering the safe access for the purpose of maintenance to, and the operation of, the equipment on his premises. (See typical example in appendix D.) Where the employer or occupier is not competent to do this, the formulation of a set of safety rules may be contracted out to a competent authority.

Where switching or maintenance work has to be done on equipment fed directly from a source of supply not under the control of the employer or occupier or the persons actually carrying out the work then special care is required.

It will. Be necessary for all parties to mutually agree procedures and methods of work in order to ensure the safety of persons carrying out the work, and for these. agreed procedures to be incorporated in the rules and procedures for the installation.

Care should also be taken to prevent equipment being worked on becoming energized due to the automatic or inadvertent starting of standby or emergency generators. In addition, the employer or occupier should ensure that precise instructions exist, based on the manufacturer's handbook for the safe handling, maintenance and testing of the equipment. The employer or occupier should also make .arrangements for monitoring to ensure that the foregoing procedures are effectively performed.

Those concerned with the maintenance of equipment should familiarize themselves with the plant it controls and report any changes which may affect the equipment. During maintenance work all personnel should pay particular attention to warning notices or instructions incorporated on the equipment or set up temporarily during the maintenance procedures.

2.3.3 Isolation and access for maintenance

2.3.3.1 General

The policy to be followed in making equipment available for maintenance should always be that it should be isolated and proved dead where possible and immediately earthed.

2.3.3.2 Procedures

No electrical conductor should be regarded as being safe unless it has been isolated and discharged to earth and, where necessary earthed at all points of supply.

Precautions should be taken to ensure that the isolated equipment cannot be re-energized from a high voltage or a lower voltage source of supply.

Voltage indicators should always be proved before and after use. It is good practice to inspect earthing devices before every use. Earthing connections including leads and associated terminations need to be of adequate capacity for the duty at the point of application.

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Barriers preventing access to enclosures containing live conductors should normally be kept locked.

Where one person isolates and another does the work, the person responsible for isolating should demonstrate effectively to the other that the equipment is in fact dead and safe and that there are adequate safeguards to prevent re energization.

Adequate quantities of suitable locks, cautionary notices and temporary barriers should be available for use to facilitate safe working and to prevent conductors from being accidentally electrically charged when persons are working thereon and also to warn of the presence of any live conductors. Such notices should be clearly legible and prominently displayed, made from durable; material and kept up-to date. Suitable precautions should be taken to identify circuits and equipment at the front and back of switchboards where such identification does not already exist. .

Any disconnectors used for isolation should be locked to prevent movement to the ‘ON’ position. Any shutters giving access to live conductors should also be padlocked in the closed position.

Equipment enclosures frequently contain, circuits having sources of supply different from that of the main circuit, such as interlocks, alarms, heating and lighting circuits, etc., and these circuits are not always isolated when the main circuit is disconnected. Conductors and terminals associated with these circuits should be shrouded where necessary to prevent accidental contact and identified with warning notices. Particular care should be taken to avoid danger from reverse energization of voltage/control transformers or the open circuiting of current transformer secondaries.

Removal and retention of fuse links or bolted links should only be used as a means of isolation when suitable precautions are taken to prevent duplicates being inserted. Contactors should never be considered as a means of isolation.

Reliance should never be placed on control circuit isolation, switching or electrical interlocks to prevent accidental or inadvertent re-energization of the main or auxiliary circuits. Where the component to be maintained is completely withdrawn from the equipment, and thus from all sources of electrical supply, that component may be regarded as a safe piece of equipment and no longer subject to the safety rules referred to in paragraph 1 of clause 4.

2.3.4 Preparing for maintenance work

Working space, entry ways and exit ways provided to apparatus and to equipment which is to be maintained should be kept clean and free from obstruction. Spare parts, tools, instruments insulating screens, insulated tools, portable earthing devices and gloves

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associated with the equipment or the work to be performed should be housed in proper receptacles provided for the purpose, and kept in proper condition.

Adequate lighting either fixed portable, or a combination of both should be provided as necessary to ensure safe access and working.

Portable electrical tools and inspection lamps should preferably be operated from a system with a voltage no greater than 110 V with either the star point of a three phase or the mid-point of a single-phase transformer low voltage winding earthed. If mains voltage portable tools need to be used, they should be of all-insulated or double insulated construction and the use of a residual current device is recommended. All portable electrical equipment should be regularly inspected and tested.

NOTE. Further advice on the safe use of portable tools is contained in HMG publication guidance note PM 32 available from HM Stationery Office.

The ingress of moisture, dirt, vermin etc. into electrical equipment can cause malfunction and danger. Care should be taken to prevent such ingress whilst work is in progress, and covers should be replaced as soon as access to the chamber is no longer required. Before final closure of any compartment is effected, a careful inspection should be carried out to make sure no foreign matter or loose material is present.

Before work is undertaken in any chamber containing high voltage conductors, tests using suitable voltage indicators should be carried out. These should include tests between .each phase and earth to ensure all conductors are dead. Voltage indicators should always be proved before and after use.

When work is being carried out with adjacent pneumatically operated or air-blast circuit breakers in service, due precautions need to be taken to protect personnel from the effects of noise caused by these circuit breakers should they operate. Where circuit breakers are fitted with silencers, no problems should, occur but if they are unsilenced consideration should be given to the use of ear protectors.

2.3.5 Fire extinguishing equipment

All personnel carrying out maintenance on equipment where there is a fire risk or using flammable materials in processes requiring flame or other sources of heat should have fire fighting appliances available for ready use. These appliances may be installed permanently by an occupier or employer for use in the premises or they may be temporary appliances provided for the period of work. Employees should be trained in the use of portable appliances and know how to summon further assistance.

If a fixed automatic fire extinguishing installation is installed, a prominent warning notice should be displayed at the entry to the protected area. The notice should

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also include instructions for preventing automatic operation when persons are working within the protected area. The prevention and restoration of automatic operation should be subject to appropriate safety procedures, for example by including a reference on the relevant permits to work. The type of fire extinguishers provided for use on or near electrical equipment should be compatible with the equipment and safe to use. Further advice on fire prevention and fire fighting may be obtained from the local Fire Prevention Officer.

2.3.6 Testing

2.3.6.1 General

Care should be taken when applying test voltages to ensure that they are the lowest value required for the purpose with the minimum current output. Where equipment is capable of storing a charge this should be safely discharged after every test.

NOTE 1: Further advice on electrical testing is available: one publication is Health and Safety Series Booklet No. HSG (13) 'Electrical testing' available from HM Stationery Office.

NOTE 2: Electrical equipment may be damaged by the application of test voltages and currents of incorrect value and polarity. Some electronic equipment is particularly vulnerable (see clause 40).

2.3.6.2 Use of test instruments (oscilloscopes, etc.)

Instruments should be of a type suitable for the measurements that are to be made so that a malfunction or the introduction of transients and/or reversed polarities into the connected circuits is avoided. The manufacturer's instructions should be observed.

An earthed instrument lead may create danger if it is applied: to an active signal circuit which is normally floating. It is recommended that the instrument casings are earthed at all times but, where the nature of test precludes this, specific care should be taken by the operator to secure his own safety and that of others by the adoption of a safe system of work. It is recommended that suitably protected test leads be used at all times.

2.3.7 Disposal of scrap

Care is needed in the disposal of removed items or materials since some give rise to health or environmental danger unless properly handled, e.g. polychlorinated biphenols (PCBs) or asbestos. In case of doubt, reference should be made to the manufacturer's instructions or the appropriate local authority

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3.0.0 Multimeter

A multimeter measures electrical properties such as AC or DC voltage, current, and resistance. Rather than have separate meters, a multimeter combines a voltmeter, an ammeter, and an ohmmeter. Electricians and the general public might use a multimeter on batteries, components, switches, power sources, and motors to diagnose electrical malfunctions and narrow down their cause.

It is a black box of electronic circuitry that allows to troubleshoot just about any type of electrical wiring or device. Simply dial the proper function and scale, touch the two test leads to the wiring or device in question and check the meter reading. Depending on the setting, the multimeter will give indication to suggest a broken connection, no power, poor connections, faulty parts and more.

The two main kinds of a multimeter are analog and digital. A digital multimeter has an LCD screen that gives a straight forward decimal read out, while an analog display moves a pointer through a scale of numbers and must be interpreted. Any multimeter will work over a specific range for each measurement. Select one that is compatible with what is required, from low-voltage power sources to high-voltage car batteries. Multimeters are specified with a sensitivity range, so make sure to choose the appropriate one.

Multimeters are handheld devices. Analog multimeters are very cheap but sometimes difficult to read accurately, especially on resistance scales. Digital output devices are much easier to read but in general, cost more than analog meters. All multimeters will have a switch that allows to select the type of test or measurement to be performed. In addition, they always have two wires with metal tips called probes, one red and one black.

As a voltmeter, a multimeter can measure the amount of AC or DC voltage flowing through a circuit. Voltage is a difference in potential energy between the two points. As an ohmmeter, a multimeter finds the resistance in a circuit, which is given in ohms. The multimeter actually passes a small amount of electricity from its own battery through the circuit to measure resistance by comparing the voltage sent out to what it receives. When used as an ammeter, the multimeter measures current flowing through a closed circuit by interrupting that circuit. The multimeter can only be connected in series, which means that all the circuit's current will flow through the ammeter's sensors

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3.1.0 Analog Multimeter

The permanent magnet moving coil analogue multimeters are based on the galvanometer invented by Arsene d’Arsonval. This device possesses a stationary permanent magnet, a moving coil, a spring, and a pointer attached to the coil. Figure below illustrates the way the equipment works. When a current flows through the coil, there is an induced force on it due to the created electromagnetic field, and the coil rotates around its central axis until the induced torque is equal and opposite to the spring torque. The rotation torque, and consequently the angle the pointer rotates is proportional to the current. The rotation angle is measured on a calibrated scale, and the amount of current flowing through the meter can be measured. The d’Arsonval movement is used

basically to measure average or DC currents and voltages

By locating the range switch and the function switch in the proper position, the desired variable may be measured in the selected scale.

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3.2.0 Digital Multimeter

Figure shows a block diagram of an electronic digital multimeter.

Note that the block diagram divides the instrument into three major sections: the SIGNAL CONDITIONING section, the ANALOG-TO- DIGITAL CONVERTER section, and the DISPLAY section.

The signal conditioning section provides a dc analog voltage, characteristic of the applied input, to the analog-to-digital converter section. This task is accomplished by the input voltage divider, current shunts, ac converter, active filter, and associated switching. The analog-to-digital (a/d) converter section changes the dc output voltage from the signal conditioning section to digital information. The a/d converter uses a voltage-to-frequency conversion technique. A dc voltage at the input of the a/d converter is changed to a frequency by the analog integrated circuit (ic). This frequency is characteristic of the magnitude and polarity of the dc input voltage. Counting of the output frequency from the analog ic is accomplished by the digital ic. The resulting count is transferred in binary format to the display section. (Binary number systems are covered in NEETS, Module 13, Introduction to Number Systems, Boolean Algebra, and Logic Circuits.) The display section takes the digital (binary) information from the a/d converter section, decodes it, and visually displays it. The decoded digital information is displayed on numerical LED readouts

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3.2.1 Voltage Measurements

Plug the black test lead into the COM jack. Plug the red test lead into the V jack.

Set the function/range switch to either DC volts in the upper left, or AC volts in the upper right.

If you do not know the approximate voltage about to be measured, use the largest voltage range available.

Connect the free ends of the red and black test leads ACROSS the device to the measured. Voltage is always measured with the meter in PARALLEL with the device.

If the LCD displays either "1." or "-1." with all other digits blank, the voltage is beyond the selected range. Use the switch to select a larger range.

Once you know the approximate voltage across the device, then use the switch to select the lowest voltage range that will still accommodate the voltage across the device. For example:

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3.2.2 Current Measurements

Turn the power off to the device and discharge any capacitors. Plug the black test lead into the COM jack.

Plug the red test lead into either the 200 mA jack for small current measurements, or the 10 A jack for large current measurements.

If you do not know the approximate current about to be measured, use the 10 A jack.

Set the function/range switch to either DC amperes in the lower right, or AC amperes in the middle right.

Break open the circuit at the point where you want to measure the current by removing one of the wires.

Connect the free end of the red test lead to one place at which the wire was attached.

Connect the free end of the black test lead to the other place at which the wire was attached.

Current is always measured with the meter in SERIES with the device. Using the current meter incorrectly will blow the fuse or damage the meter Reapply the power to the device.

If the LCD displays either "1." or "-1." with all other digits blank, the current is beyond the selected range. Use the switch to select a larger range.

Once you know the approximate current through the device, then use the switch to select the lowest current range that will still accommodate the current through the device.

Turn the power off to the device before removing the meter from the circuit.

3.2.3 Resistance Measurements

Turn the power off to the device and discharge any capacitors! Plug the black test lead into the COM jack.

Plug the red test lead into the V jack.

Set the function/range switch to ohms in the lower left.

If you do not know the approximate resistance about to be measured, use the largest range available.

Connect the free ends of the red and black test leads ACROSS the device to the measured. Resistance is always measured with the meter in PARALLEL with the device.

If the LCD displays either "1." or "-1." with all other digits blank, the

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Once you know the approximate resistance of the device, and then use the switch to select the lowest range that will still accommodate the resistance of the device.

Important note: The most common mistake when using a multimeter is not switching the test leads when switching between current sensing and any other type of sensing (voltage, resistance). It is critical that the test leads be in the proper jacks for the measurement you are making.

3.3.0 Digital vs. Analog Multimeters

Digital multimeters have LCD readouts, do audible continuity testing. Some digital multimeters also feature auto-ranging and overload protection and other advantages analog multimeters lack.

Analog multimeters have multiple scales on the dial , a moving needle and many manual settings on the function switch. It is a tricky spotting the correct scale to read on the dial, and sometimes have to multiply the reading by 10 or 100 to get your final value

3.4.0 Safety Precautions

Be sure the test leads and rotary switch are in the correct position for the desired measurement.

Never use the meter if the meter or the test leads look damaged. Never measure resistance in a circuit when power is applied.

Never touch the probes to a voltage source when a test lead is plugged into the 10 A or 300 mA input jack.

To avoid damage or injury, never use the meter on circuits that exceed 4800 watts. Never apply more than the rated voltage between any input jack and earth ground Be careful when working with voltages above 60 V DC or 30 V AC rms. Such voltages pose a shock hazard.

Keep your fingers behind the finger guards on the test probes when making measurements.

To avoid false readings, which could lead to possible electric shock or personal injury, replace the battery as soon as the battery indicator appears.

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4.0.0 Insulation resistance tester (Megohmmeter)

The Insulation resistance tester (IRT) also known as a megohmmeter, is a portable instrument used to measure insulation resistance. It is a lightweight, simple and in minutes can help to determine the damaged installation. Checking the integrity of insulation is not only a good idea for a new installation, it’s a tremendous tool in ongoing maintenance, allowing to spot a problem wiring/equipment, before it creates arcing and damages the equipment or shuts everything down.

The principle of operation of IRTs is as basic as Ohm’s Law: V=IR or R=V/I. The tester generates a known dc voltage (250 V, 500 V, 1k V or higher), chosen by the user, and measures the leakage current from the conductor through the insulation. The resistance is then calculated. The better the insulation, the lower the leakage current and the higher the amount of resistance present.

For example, if 500 V is applied and 0.5 mA measured, then R=1 MΩ. If only one hundredth of that current, 5 μA, is measured, then R=100 MΩ

The newest generation of IRTs is microprocessor-based and battery-powered. They are more precise than the older hand-cranked analog testers.

4.1.0 Analog Megohmmeter

It consists of a hand-driven DC generator and a direct reading ohm meter. A simplified circuit diagram of this instrument is shown in Figure below.

The moving element of the ohm meter consists of two coils, A and B, which are rigidly mounted to a pivoted central shaft and are free to rotate over a C-shaped core. These coils are connected by means of flexible leads. The moving element may point in any meter position when the generator is not in operation.

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will tend to set itself at right angles to the field of the permanent magnet. With the test terminals open, giving an infinite resistance, no current flows in Coil A. Thereby, Coil B will govern the motion of the rotating element, causing it to move to the extreme counter-clockwise position, which is marked as infinite resistance.

Coil A is wound in a manner to produce a clockwise torque on the moving element. With the terminals marked "line" and "earth" shorted, giving a zero resistance, the current flow through the Coil A is sufficient to produce enough torque to overcome the torque of Coil B. The pointer then moves to the extreme clockwise position, which is marked as zero resistance. Resistance (R1) will protect Coil A from excessive current flow in this condition.

When an unknown resistance is connected across the test terminals, line and earth, the opposing torques of Coils A and B balance each other so that the instrument pointer comes to rest at some point on the scale. The scale is calibrated such that the pointer directly indicates the value of resistance being measured.

4.2.0 Digital Megohmmeter

There are an extensive and a versatile range of hand-held insulation testers designed to extend the capability of insulation testers beyond anything available on the market today.

In addition to extremely high-sensitivity insulation resistance measurements (200 GΩ), these instruments offer complete multimeter testing capability and the ability to view the insulation measurement in terms of leakage current (µA). The top-of-the- range models also include data storage and download capability. The end user can now carry a single instrument to the test site rather than multiple instruments.

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Some of the common features are:

¾ Lightweight. Tough and robust

¾ Shrouded safety terminals with right angled test lead connector. ¾ Hands-free operation

¾ Voltage ranges of 250, 500, and 1000 V. ¾ Resistance measurement range of 200 GΩ ¾ Combined insulation & Continuity tester ¾ Default Voltmeter

¾ Battery condition test

¾ Automatic discharge of capacitive circuits after test ¾ Auto shut off

Some of the top model’s advance features are: ¾ Auto ranging

¾ Selectable and programmable test voltage range (40 to 5000V) ¾ Automatic calculation of Polarisation Index(PI)

¾ Direct measurement and display of Capacitance & Leakage current ¾ Programmable test run time and PI ratio time

¾ Automatic test inhibition (if live sample > 25V) ¾ RS-232 interface for direct printing of results

¾

128 kB memory for storing field test data

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4.3.0 Insulation resistance test

Insulation resistance tests give an indication of the condition of insulation, particularly with regard to moisture and dirt. The actual value of the resistance varies greatly in different types of machines, depending on the type, size, voltage rating, etc. The principal worth of such measurements, therefore, is in the relative values of insulation resistance of the same apparatus taken under similar conditions at various times. Such tests usually reveal how well the machine has been maintained.

Measuring insulation resistance is rather straightforward. Identify any two points between which there is insulation and make a connection with a megohmmeter. Take a measurement; the measured value represents the equivalent resistance of all the insulation that exists between the two points and any component resistance that might also be connected between the two points.

Megohmmeters are available in several varieties. Some are powered by a hand-cranked generator, while others are battery powered. Some use power supply voltages as low as 50V, but the most common is 50OV, with some going as high as 10,OOOV. The power supply, in all cases, is DC.

4.3.1 Components of DC leakage current

When a megohm. test is made, three components of current flow. The insulation between the two connection points can be thought of as a dielectric, thus forming a capacitance. A phenomenon known as “dielectric absorption” occurs whereby the dielectric soaks up current and then releases it when the potential is removed. This is in addition to the current that charges and discharges the capacitance, and it occurs much more slowly. It is dependent on the nature of the dielectric. Two types of items where this is of concern are capacitors and motors. Such current is referred to as dielectric absorption current.

The current required to charge whatever capacitance is present is known as “charging current” Like the dielectric absorption current, it decays exponentially to zero,

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but much more quickly. It is this current which in most cases determines how long it takes to make an accurate megohm measurement. When the reading appears to stabilize, it means that the charging current has decayed to a point where it is negligible with respect to the leakage current.

The current that flows through the insulation is the leakage current. The voltage across the insulation divided by the leakage current through it equals the insulation resistance. Thus, to accurately measure insulation resistance, we must wait until the dielectric absorption current and the charging current have decayed to the point where they are truly negligible with respect to the leakage current.

The total current that flows is the sum of the three components just mentioned. It decays exponentially from an initial maximum and approaches a constant value which is the leakage current alone. The megohm reading is dependent on the voltage across the insulation and the total current. It increases exponentially from an initial

4.3.2 Determining The Polarization Index

Knowing the polarization index of a motor, generator or transformer can be useful in appraising the fitness of the machine for service. The index is calculated from measurements of the winding insulation resistance.

Before measuring the insulation resistance, remove all external connections to the machine and completely discharge the windings to the grounded machine frame.

Proceed by applying either 50OVDC or 1,OOOVDC between the winding and ground using a direct-indicating, power-driven megohmmeter. For machines rated 500V and over, the higher value is used. The voltage is applied for 10 minutes and kept constant for the duration of the test.

The polarization index is calculated as the ratio of the 10-minut value to the one-minute value of the insulation resistances measured consecutively:

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Polarization index = resistance after 10 minutes resistance after one minute

The recommended minimum value of polarization index for AC and DC motors and generators is 2.0. Machines having windings with a lower index are less likely to be suited for operation.

The polarization index is useful in evaluating windings for: ¾ Buildup of dirt or moisture

¾ Gradual deterioration of the insulation (by comparing results of tests made earlier on the same machine)

¾ Fitness for overpotential tests ¾ Suitability for operation

4.3.3 Correction for Winding Temperature

One important point to remember is that the value of insulation resistance decreases as the temperature of the insulation increases. This is just the opposite of the effect for conductor resistance. In order to gather meaningful information that can be used for comparison purposes, we should ideally carry out periodic tests with the winding at the same temperature conditions. If this is not possible, it is necessary to measure the actual temperature of the winding under test and then correct the resistance reading to a standard temperature, usually 20oC.

If we measured the insulation resistance to be, say 100 MW at a temperature of 30oC, we should multiply the result by the correction factor of 1.98 shown on the chart below. This implies that at a temperature of 20oC the insulation resistance would increase to 198 MW. These correction factors will normally be provided by the equipment manufacturer. The main point here is that all readings, past, present and future should be compared on the same basis

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4.3.4 Insulation Contamination

In order to obtain reliable test figures, windings should be free of dirt and moisture as both of these contaminants result in a lower value of resistance being indicated. If a machine has recently been taken out of service, it is likely to be hot and therefore free from moisture.

However, the windings may be quite dirty from dust and oil in the atmosphere. Conversely, if the machine has been out of service for some time, the winding insulation may well have absorbed a certain amount of moisture. Indeed, if the insulation resistance is indicated as low, it may be necessary to dry out the windings. All of these items must be taken into consideration when assessing the reliability of insulation resistance readings

4.4.0 Insulation resistance test methods

When testing the insulation resistance in electrical equipment, any of the following methods can be employed.

¾ Spot reading ¾ Step voltage ¾ Time resistance test

Testing insulation in electrical systems is a critical step improving safety and longer system life.

4.4.1 Spot testing

This is a short time resistance test. The megohmmeter is connected directly across the equipment being tested and a test voltage is applied for about 60 secs. In order to get a stable insulation resistance reading in that time, the test is performed on low capacitance equipment.

This is a simple and quick test to indicate the instantaneous condition of insulation. Mostly used as part of the commissioning process for a new installation.

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4.4.2 Step voltage test

Various test voltage in steps of (in increasing order), usually for the same period of time (60 Secs). The insulation values are recorded on the graph. The insulation is exposed to increased electrical stress that can reveal information about flaws in the insulation such as pinholes, physical damage or brittleness.

4.4.3 Time resistance test

This test is carried out to compare the absorption characteristic of contaminated insulation with the absorption characteristic of good insulation. The test voltage is applied for the 10 mins and after every 10 secs the data is recorded for first one minute and then after every one minute. The interpretation of the slope of the plotted graph determines the condition of the insulation.

The polarisation index is another test in this category for determining the quality of insulation. This is discussed in earlier part of this chapter.

4.5.0 Safety Precautions

Be sure the test leads and rotary switch are in the correct position for the desired measurement.

Never use the meter if the meter or the test leads look damaged. Never measure insulation resistance in a energised circuit.

Make sure that the systems under test have been completely discharged to ground.

Never touch the probes to a voltage source when a testing is in progress Test voltage should not exceed the equipment manufacturer’s recommended

voltage for insulation testing.

Ensure that equipment under test is discharged to ground after the test, if the equipment do not have auto discharge feature

Be careful when working with voltages above 60 V DC or 30 V AC rms. Such voltages pose a shock hazard.

Keep your fingers behind the finger guards on the test probes when making measurements.

To avoid false readings, which could lead to possible electric shock or personal injury, replace the battery as soon as the battery indicator appears.

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4.6.0 Insulation resistance (IR) test on Cables

Prior to testing required permits must be secured. All safety precautions should be observed while testing. Cable must be discharged and disconnected form the equipment in the field.

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4.7.0 Insulation resistance (IR) test on Transformer

Prior to testing required permits must be secured. All safety precautions should be observed while testing.

4.8.0 Insulation resistance (IR) test on Motor/Generator

Prior to testing required permits must be secured . All safety precautions should be observed while testing

The easiest test that prevents the most failures is the “insulation resistance measurement”, It applies DC voltage, usually 500 or 1000 volts, to the motor and measures the resistance of the insulation between the windings and earth (frame).

Before testing the motor or Generator lift the rotor brushes (whereever applicable), earth the stator terminals, frame and shaft. Discharge the field winding by earthing. Then remove the field winding from earth and measure the field winding insuation to earth.

A minimum resistance to earth at 40 degrees C ambient of 1 megohm per kV of rating plus 1 megohm may be acceptable. Medium size motors in good condition will generally have megohmmeter readings in excess of 50 megohms. Low readings may indicate a worse condition of insulation caused by contamination from moisture, oil or conductive dirt or deterioration from age or excessive heat.

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Measuring insulation resistance is rather straightforward. Identify any two points between which there is insulation and make a connection with a the test instrument is often referred to as a Megger, after the manufacturer’s trademark. Take a measurement; the measured value represents the equivalent resistance of all the insulation that exists between the two points and any component resistance that might also be connected between the two points.Megger s are available in several varieties. Some are powered by a hand-cranked generator, while others are battery powered. Most common is 500V output, with some going as high as 10,000V. The power supply, in all cases, is DC.

1kV 500V 250V 100V 50V O FF V Ω Ω k TE ST Ω MΩ ZERO AVO M EGGER XXXX

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5.0.0 Clamp on meter

A clamp meter (clamp-on ammeter) is a type of ammeter which measures electrical current without the need to disconnect the wiring through which the current is flowing.

Clamp meters are also known as tong testers or Amprobes (after Amprobe Instrument Company, one of the first vendors of such devices).

The most common forms of clamp meter are: A probe for use with a multimeter. A self-contained unit.

A built-in part of a specialised multimeter used by electricians.

In order to use a clamp meter, the probe or clamp is opened to allow insertion of the wiring, and then closed to allow the measurement. Only one conductor is normally passed through the probe, if more than one conductor were to be passed through then the measurement would be a vector sum of the currents flowing in the conductors and could be very misleading depending on the phase relationship of the currents. In particular, if the clamp were to be closed around a mains extension or similar cord, no current will be measured at all as the current flowing in one direction will cancel that flowing in the

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other direction.

Only one (1) conductor can be measured at a time, and the cable can either be bare or insulated. The current in the conductor to be measured is (carefully) segregated from other current-carrying conductors, and shifted enough so that the jaws of the clamp-on ammeter can be opened, slipped around the cable, and then closed. As soclamp-on as the jaws close, a clear and accurate reading is registered on the scale. The jaws are insulated, and the Bakelite handle and shield protect the technician from shock.

5.1.0 Theory of operation of AC Clamp on meter

The meter is operated by the magnetic field set up by the current. Basic construction of the meter is a clamp on current probes and the ammeter (Analog or digital) connected to it.

The clamp on current probe works on the principle of current transformer. The conductor is the primary coil of a current transformer

The clamp on probe are composed of permalloy split core with the winding coil which connects to the meter (Analog or digital) circuit. First the conductor current determines the strength of the magnetic field around the permalloy core. Then at the end of the winding, the magnetic field induces secondary out-put which drives the current. through the meter (Analog or digital) connected. The meter is calibrated to indicate the current in the primary.

A clamp-on current probe, shown in the above diagram, converts the primary current of the conductor to a current output whose value depends on number of turns of secondary winding (N2). If N2 is 1000 turns, the output current is 1/1000 of the primary current, which can be expressed as 1 Milliampere per Ampere. Such a clamp is referred to as having a ratio of 1000:1. The output of this current clamp can be read by any AC ammeter (Analog or digital) whose input impedance is compatible with the specifications of the current clamp.

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5.2.0 Theory of operation of AC/DC Clamp on meter

More modern designs of clamp-on ammeters utilize a small magnetic field detector device called a Hall-effect sensor to accurately determine field strength. The two matched sensors provide an output signal which is independent of the location of the current conductor in the clamp opening. The conductor does not have to be exactly at the center of the opening. A battery-operated circuit is required to provide the excitation and amplification of the signal generated by the HALL-EFFECT sensor

Hall effect principle : AC/DC current sensing is achieved by measuring the strength of the magnetic field created by a current carrying conductor in a semiconductor chip using Hall effect principle. When a thin semiconductor is placed at right angle to a magnetic field (B), and a current (Id) is applied to it, a voltage (Vh) is developed across the semiconductor. This voltage is known as the Hall voltage, named after the US scientist Edwin Hall.

When the Hall device drive current is held constant, the current is directly proportional to the current in a conductor. Thus, the hall output voltage (Vh) is representative of that current.

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Some clamp-on meters contain electronic amplifier circuitry to generate a small voltage proportional to the current in the wire between the jaws, that small voltage connected to a voltmeter for convenient readout by a technician. Thus, a clamp-on unit can be an accessory device to a voltmeter, for current measurement.

5.3.0 Specifications of Digital Clamp on meters

 AC Current : It is the measuring value of the alternating current taken by the load of a clamp on meter

 AC Voltage : It is defined as the alternating voltage measured by the clamp on meter.

 DC Current : It is the measuring value of the direct current read by the clamp on meter

 DC Voltage : It is defined as the direct voltage measured by the clamp on meter

 Resistance : It is the resistance offered by the clamp on meter to the current flow

 Frequency range : It is the range of frequency at which the current or voltage is measured.

 Distortion factor : It is defined as a measure of non linear distortion.  Total harmonic distortion : It is defined as the ratio of the sum of the

powers of all harmonic frequencies above the fundamental frequency to the power of the fundamental frequency. It is usually referred to as THD. THDandis measured in dB

 Display type : It is the type of electronic display where the measured values are monitored and graphed.

 Crest factor : It is the ratio of the peak amplitude value to its RMS value.  Clamp jaw size : It refers to the diameter value of the opening jaw.

5.4.0 Advantages of Digital Clamp on meter

Fast measurements Precise measurement Less meter loading

5.5.0 Advantages of Digital over Conventional Type

Digital clamp on meters are more advantageous than the conventional clampmeters. Because in the conventional type, the current wave obtained is of sinusoidal nature. The current value is measured generally in RMS units. If these factors are not

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taken into consideration, it is of less use with non sinusoidal load applications such as fluorescent lamps, modern computers, electronic equipment, or high-intensity discharge lamps.

5.6.0 Applications of Digital Clamp on meters

Motor drives

Electric vehicles

Electricity supply industry(ESI) Automotive diagnostic plants Electrochemical plants Power supplies

Welding equipment

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5.8.0 Safety Precautions

To avoid possible electric shock or personal injury, and to avoid possible damage to the Meter or the equipment under test, adhere to the following practices:

Avoid working alone as far as possible, and render the assisstance. Never use the Meter on a circuit with voltages higher than specified. Never use the Meter on a circuit with frequency higher than specified, the

meter could be damaged.

Do not use the Meter or test leads if they look damaged.

Use extreme caution when working around bare conductors or bus bars. Contact with the conductor could result in electric shock.

Read the manufacturer’s instructions and safety sheet before use and follow all safety instructions.

Use the Meter only as specified in the instruction manual; otherwise, the Meter’s safety features may be impaired.

Use caution when working with voltages above 60 V dc or 30 V ac. Such voltages pose a shock hazard.

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Before using the Meter, inspect the case. Do not use the Meter if it is damaged. Look for cracks or missing plastic. Pay particular attention to the insulation around the connectors.

Verify the Meter’s operation by measuring a known current/voltage. Do not use the Meter if it operates abnormally. Protection may be impaired. When in doubt, have the Meter serviced.

Do not apply more than the rated current or voltage, as marked on the Meter.

Use the proper terminals, function, and range for your measurements. Do not operate the Meter with the case (or part of the case) removed. When servicing the Meter, use only replacement parts recommended by

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6.0.0 Earth tester, Loop tester & Residual Current Device

(RCD) tester

A practical earth electrode that provides a low ground resistance is not always easy to obtain. The metallic body in the earth is often referred to as an electrode even though it may be a water-pipe system, buried strips or plates, or wires. Such combinations of metallic bodies are called a grid. The earth resistance is the resistance to current from the electrode into the surrounding earth.

To appreciate why earth resistance must be low, you need only use Ohm’s Law: E = R x I where E is volts; R, the resistance in ohms; and I, the current in amperes. Assume that you have a 4000-V supply (2300 V to ground) with a resistance of 13 Ω (see Fig. below). Now, assume that an exposed wire in this system touches a motor frame that is connected to a grounding system which has a 10-ohm resistance to earth.

By Ohm’s Law, there will be a current of 100 A through the fault (from the motor frame to the earth). If someone happen to touch the motor frame and are grounded solidly to earth, could be subjected to 1000 V (10 Ω x 100 A). This may be more than enough to kill a person instantly. If, however, the earth resistance is less than 1 Ω, the shock person would get could be under 100 V (1 x 100) and probably survive that shock.

Earth resistivity has an important bearing on electrode resistance, as does the depth, size and shape of the electrode. In this module, the principles and method of testing and use of earth resistance tester are covered. This applies to lightning arrester installations as well as to other systems that require low resistance ground connections. Such tests are made in power-generating stations, electrical-distribution systems, industrial plants, and telecommunication systems.

Also an Earth loop impedance testing is essential since if a live conductor is accidentally connected to an earth conductor in a faulty appliance or circuit, the resulting short-circuit current to earth can easily be high enough to cause electric shock or generate enough heat to start a fire. Normally, the fuse will blow or another circuit protection device will trip, but a situation may arise where the actual short-circuit current in a faulty

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installation is of insufficient level and the protection device would thus take too long to activate. The delay can be disastrous for life and property. It is therefore necessary to know if the impedance of the path that any fault current would take is low enough to allow sufficient current to flow in the event of a fault and that any installed protective device will operate within a safe time limit. The earth loop impedance of each individual circuit a path from the point of use back to the incoming supply connection point. As measurement of circuit loop impedance is made with the supply normally on, precautions must be taken to avoid the possibility of electric shock and danger to personnel working in the vicinity of the circuit under test.

In IEC 60364, fault loop testing falls under the category of ‘Verifying protection by automatic supply disconnection’. This covers verification of the effectiveness of protective measures (such as test on RCD), and the test methods applied to measure the fault loop impedance.

Conventional techniques for measuring loop impedance can often trip RCDs, preventing further measurement. Often the only way around this is to “bridge” the RCD or replace the RCD with an equivalent rated MCB for the duration of the test – both of which are potentially dangerous and time consuming practices. To overcome this manufacturer’s of earth loop tester have applied innovative technology to ensure that both electromechanical and electronic type RCDs do not trip during earth loop impedance measurements.

An advanced range of combined loop and RCD testers is available and it is designed to fully test RCDs and measure loop impedance and prospective short circuit current, (PSCC), on single and three phase systems rated up to 300V ac r.m.s. to earth.

6.1.0 Earth resistance

Resistance to current through an earth electrode actually has three components. 1. Resistance of the electrode itself and connections to it.

2. Contact resistance between the electrode and the soil adjacent to it. 3. Resistance of the surrounding earth.

Electrode Resistance: Rods, pipes, masses of metal, structures, and other devices are commonly used for earth connections. These are usually of sufficient size or cross-section that their resistance is a negligible part of the total resistance.

Electrode-Earth Contact Resistance: This is much less than you might think. If the electrode is free from paint or grease, and the earth is packed firmly, contact resistance is negligible. Rust on an iron electrode has little or no effect; the iron oxide is readily soaked with water and has less resistance than most soils. But if an iron pipe has